Single crystal articles having controlled secondary crystallographic orientation

Abstract

Alignment of the [001] crystal axis of a face centered cubic metal with the primary z axis of a single crystal article provides good thermal fatigue resistance along the z axis, and minimizes cracking transverse to the axis. However, significant cracking is still observed parallel the z axis in severe applications. This cracking can be reduced by controlling the secondary crystallographic orientation (i.e., orientation of crystal axes within x-y planes transverse to the z axis), to make the [110] crystal axis tangent to the article surface in the region most prone to thermal fatigue cracking. Algorithims derived from empirical relationships enable calculation of the orientation likely to produce improved fatigue resistance. More durable single crystal gas turbine blades result when the [110] crystal axis is made tangent to the blade surface in the critical crack prone regions just behind the leading edge of the airfoil at about 40-80% of the airfoil span. A representative improved gas turbine blade will have a secondary orientation angle α of -10 to +20 degrees, where α is the angle between the [100] crystal axis and the y axis.

We claim: 
     
       1. A single crystal article with orthogonal x, y, and z axes, made of a face centered cubic crystal structure metal, having a [001] crystal axis aligned within at least about 20 degrees of the z axis, characterized by a [110] crystal axis of the metal aligned with a tangent to the surface of the article in a region which is prone to thermal fatigue cracks which run in the z axis direction. 
     
     
       2. The article of claim 1, shaped as a hollow aircooled gas turbine component having an airfoil portion, made of a superalloy, characterized by a [110] crystal axis aligned with a tangent to the surface of the airfoil on its convex (suction) side, the thermal fatigue crack prone region lying between 40-80 percent of the airfoil span. 
     
     
       3. The article of claim 2 characterized by a thermal fatigue region bounded in the forward and rearward directions by loci of intercept points, with the surface, of radii of the leading edge circles of the x-y planar cross sections lying between the 40-80 percent span, the radii at each planar section comprising a first radius rotated about 30 degrees forward from the leading edge circle diameter which is perpendicular to the tangent to the mean camber line of the planar section, at the center of the leading edge circle, and a second radius rotated about 45 degrees rearward from the said diameter. 
     
     
       4. The article of claim 3 further characterized by having the first radius angle at 25 degrees and the second radius angle at 35 degrees. 
     
     
       5. The article of claim 1, shaped as a hollow air-cooled gas turbine component having an airfoil portion, made of a superalloy, characterized by a [110] crystal axis aligned with a tangent to the surface of the airfoil on its concave (pressure) side, the thermal fatigue prone region lying between 40-80 percent of the airfoil span. 
     
     
       6. The article of claim 5 characterized by a thermal fatigue prone region bounded in the forward and rearward directions by loci of intercept points, with the surface, of radii of the leading edge circles of the x-y planar cross sections lying between 40-80 percent span, the radii at each planar section comprising a first radius coinciding with the leading edge circle diameter which is perpendicular to the tangent to the mean camber line of the planar section, at the center of the leading edge circle, and a second radius rotated about 35 degrees rearward from said diameter. 
     
     
       7. The article of claims 3, 4, or 5 characterized by the thermal fatigue prone region lying between 50-70 percent span. 
     
     
       8. A gas turbine component with orthogonal x, y, and z axes, having a portion shaped as an airfoil made of face centered cubic crystal structure superalloy, the portion having a [001] crystal axis within 20 degrees of the z axis, characterized by the projection in the x-y plane of a [100] crystallographic axis lying within 10 degrees negative and 30 degrees positive rotational angle of the y axis nearest the trailing edge, where positive angular rotation is about the z axis toward the convex side of the airfoil portion. 
     
     
       9. The component of claim 6 further characterized by the projection of the [100] crystallographic axis lying between 0 degrees and 20 degrees positive. 
     
     
       10. The method of producing improved fatigue resistance in an article made of a material having a single crystal portion with a face centered cubic crystal structure and a [001] z axis orientation characterized by controlling the secondary crystal orientation in the region of the single crystal portion which is most prone to thermal fatigue cracking using steps which include (1) selecting at least two angular orientations of the [100] axis with respect to the y axis, said orientations defined by the angle α between the [100] axis and the y axis;   (2) calculating for each angle α, such as by finite element analysis, the local anisotropic damage factor φ for a multiplicity of locations on the surface of the article where φ is inversely proportional to the fatigue life and is represented by   φ=A.sub.1 exp (A.sub.2 T)(λ·Δε).sup.A.sbsp.3        where ##EQU8##  and where ψ is the angle between the [100] crystal axis and the tangent to the surface at location; E.sub.[lmn]  and E.sub.[100]  are the elastic moduli, respectively, of the crystal axis [lmn] aligned with the y axis for a particular α, and the crystal axis [100]; and   (3) providing in the article an orientation which is associated with a low value of φ, compared to the value of φ produced by at least one other orientation.   
     
     
       11. The method of producing improved thermal fatigue resistance in an article made of a material having a single crystal portion with a face centered cubic crystal structure and a [001] z axis orientation characterized by controlling the secondary crystal orientation to align the [110] crystal axis with the tangent to the surface in the most thermal fatigue crack prone region.